1. Field of the Invention
The present invention relates to magnetic recording materials and, in particular, to those utilizing a new binder composition which imparts to the recording material superior surface properties, improved abrasion resistance and improved adhesion between the recording layer and the substrate.
2. Description of the Prior Art
As the binder used in the magnetic recording layer, a variety of thermoplastic resins and thermosetting resins and mixtures thereof have been conventionally utilized.
As thermoplastic resins, those are used which have a softening point below about 150.degree. C., an average molecular weight of from about 10,000 to 200,000, and a degree of polymerization of from about 200 to 1,000, including, e.g., copolymers of vinyl chloride and vinyl acetate, copolymers of vinyl chloride and vinylidene chloride, copolymers of vinyl chloride and acrylonitrile, copolymers of an acrylic ester and acrylonitrile, copolymers of an acrylic ester and vinylidene chloride, copolymers of an acrylic ester and styrene, copolymers of a methacrylic ester and acrylonitrile, copolymers of a methacrylic ester and vinylidene chloride, copolymers of a methacrylic ester and styrene, urethane elastomers, poly(vinyl fluoride), copolymers of vinylidene chloride and acrylonitrile, copolymers of butadiene and acrylonitrile, polyamide resins, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), copolymers of styrene and butadiene, polyester resins, copolymers of chlorovinylether and an acrylic ester, amino resins, thermoplastic resins comprising various synthetic rubbers, and mixtures of the above cited resins.
The use of these resins is described in the following Japanese Patent Publications Nos. 6877/1962, 12528/1964, 19282/1964, 5349/1965, 20907/1965, 9463/1966, 14095/1966, 16985/1966, 6428/1967, 11621/1967, 4623/1968, 15206/1968, 2889/1969, 17947/1969, 14500/1969, 18573/1972, 22063/1972, 22064/1972, 22068/1972, 22069/1972, 22070/1972, 27886/1972, etc. and U.S. Patents 3,144,352, 3,419,420, 3,499,789, 3,713,887, etc.
Typical thermosetting resins are those which have a molecular weight of less than about 200,000 as a coating mixture and which form a three-dimensional network after coating and drying to give an infinitely large molecular weight structure as the result of condensation and/or addition reactions. Moreover those are preferably used which tend neither to soften nor melt until they begin to thermally decompose. Practical examples include, for example, phenol resins, epoxy resins, polyurethane curable resins, ureaformaldehyde resins, melamine-formaldehyde resins, alkyd resins, silicone resins, reactive acrylic resins, epoxy-polyamide resins, nitrocellulose-melamine resins, a mixture of a high molecular weight polyester resin and an isocyanate prepolymer, a mixture of a copolymer containing a salt of methacrylic acid and a di-isocyanate prepolymer, a mixture of a polyester-polyol and a poly-isocyanate, urea-formaldehyde resins, a mixture of a low molecular weight glycol, a high molecular weight diol and triphenylmethane tri-isocyanate, polyamide resins and mixtures thereof.
The use of these resins are described in, for example, the following Japanese Patent Publication Nos. 8103/1964, 9779/1965, 7192/1966, 8016/1966, 14275/1966, 28023/1967, 14501/1970, 24902/1970, 13103/1971, 22065/1972, 22066/1972, 22067/1972, 22072/1972, 22073/1972, 21045/1972, 28048/1972, 28922/1972, etc., and U.S. Patents 3,144,353, 3,320,090, 3,437,510, 3,597,273, 3,781,210, 3,781,211, etc.
When, however, any of these binder formulations was applied to a recording tape, especially to a video tape or an electronic computer tape, dropout tended to frequently occur due to the abrasive peel-off of the magnetic recording layer. Further, urethane containing binder formulations could not be used for video tapes because of their poor dispersion property and an inferior surface property.
In the past alkyd resins were usually cured by either a metal soap, a melamine-formaldehyde resin, etc. Such curing proceeded rather slowly and was often incomplete, and thus the mechanical strength of the tape was sufficient only after a prolonged period of storage following the manufacture. When such a tape was subjected to recording immediately after manufacture, the magnetic layer was easily worn off, causing recording head choking.